Method and arrangement to convert an optical data signal from a multi mode fibre to a corresponding signal into a signal mode fibre

ABSTRACT

The present invention discloses a method and a device for conversion of an optical data signal from a multimode fibre (1), which except for normal data signals may contain interruptions and an overmodulated low frequency signal, to a corresponding signal which is transmitted further into a single mode fibre (8) by a laser (7). An electric signal from an optical receiver (2) partly directly and partly through a low pass filter (4) is coupled to a comparator (5) the output of which modulates the laser (7). Upon detection of an optical interruption the low pass filter (4) is influenced such that its output signal is stored in a memory device, and the laser is influenced by diversion of the laser current, simultaneously as an analogue value in the control loop of the laser is stored. When the interruption ends both the laser transmitter (7) and the low pass filter (4) continue to operate as before on the basis of their stored analogue values.

FIELD OF THE INVENTION

The present invention relates to a method and a device to convert anoptical data signal from a multimode fibre to a corresponding signalwhich is transmitted further into a single mode fibre by means of alaser. In particular, the present invention relates to converting anincoming optical signal into electric signals which in turn areconverted into an optical output signal. The incoming signal isgenerated by a light emitting diode meaning that it is opticallyincoherent. The outgoing signal is assumed to originate from a laser.The incoming optical data signal comprises "ones" (high optical level)and "zeros" (a lower optical level) and may also contain interruptions(a further lower optical level) and an overmodulated low frequencysignal.

DESCRIPTION OF PRIOR ART

For many years it has been known that an incoming optical data signalmay be recreated to a similar outgoing signal by having such signal gothrough a fibre optical regenerator which converts the incoming opticalsignal into electric signals, which in turn are converted into anoptical output signal. In those instances where the type of incomingoptical signal is known, i.e. such parameters as bit rate, encoding,etc. are known, it is possible to design an arrangement which recreatesthe incoming optical signal into a corresponding optical output signal.The drawback of such an arrangement is that if the optical input signalis different from the signal for which the arrangement was designed, itis likely that the conversion will not work. Further, with the exceptionof a data signal, there is no such arrangement known which manages totransfer an overmodulated low frequency signal and opticalinterruptions.

It should be noted that there are known fibre optical amplifiers which,without electric intermediary steps, may recreate an arbitrary opticalinput signal. Such fibre optical amplifiers require that the incomingoptical signal be coherent. This is not a prerequisite of the presentinvention.

SUMMARY OF THE INVENTION

The technical problem of the present case is to be able to transfer anormal digital optical data signal, which additionally may containoptical interruptions, as well as an overmodulated low frequency signal,without detailed knowledge of the type of optical signal.

FIG. 1 demonstrates an example of such an optical signal. Note that thevertical scale is graded in dBm (dB related to I mW) to better manifestwhat a so called optical interruption implies. Due to practical reasons,FIG. 1 is not produced fully according to scale. If, for example, thedata rate is 10 Mbit/s the overmodulated low frequency signal may onlybe of the order 5 Khz, i.e., several orders of magnitude less that thedata rate, which is not obvious from FIG. 1. The optical interruptionmay typically vary from a few hundred μs to several seconds.

In optical systems making use of interruptions, e.g., fibre opticalnetwork versions of a Token ring, Ethernet or Token bus, it is requiredthat the optical power level, during the interruption, be very low,e.g., less than -40 dBm. Conventional laser transmitters are not able totransfer such interruptions since a laser transmitter is normally biasedto its so called laser threshold, which may typically be at -25 dBm;i.e. about 30 times (15 dB) higher in power than an interruption. Thepresent invention contains a laser transmitter which overcomes theseconcerns.

The most essential advantage of the present invention is that it ispossible to transfer in a simple manner an unknown type of normaldigital optical data signal which additionally may contain opticalinterruptions and an overmodulated low frequency signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph which demonstrates an example of an optical inputsignal and an optical output signal of the arrangement according to thepresent invention;

FIG. 2 is a block diagram which demonstrates an arrangement of thepresent invention;

FIG. 3 is an embodiment of a low pass filter 4 of the arrangement ofFIG. 2;

FIG. 4 is an embodiment of resistance 14 of the arrangement of FIG. 3;

FIG. 5 is an embodiment of a laser transmitter 7 of the arrangement ofFIG. 2; and

FIG. 6 is an embodiment of an integrator 25 of the arrangement of FIG.5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 demonstrates an arrangement of the present invention in the formof a block diagram. An optical input signal arrives through an opticalfibre 1. Such optical signal is transferred to a DC-coupled receiver 2which transforms the optical signal into a proportional electric signalwhich is available at node 9. The DC-coupling of the receiver signifiesthat the signal average of the electric signal is directly proportionalto the optical average power.

The output signal from the receiver 2 is connected to an interruptiondetector 3 having a given preset interruption detection delay. Beforeproceeding with a discussion of detector 3, it will be helpful tofurther discuss the signal shown in FIG. 1. In particular, it should benoted that if the overmodulated low frequency signal is disregarded, theoptical input signal is designated in FIG. 1 as having three levels,namely, "level one", "level zero" and "interruption level". Level "one"corresponds to the highest optical level and level "zero" is typically10-20 dB (10-100 times) below level "one". The interruption level istypically 30-40 dB below level "one".

It should also be noted that in technical specifications of optical datasignals it is only the proportion between "level one" and "level zero"which is specified as being greater than a particular quantity in dB.The same is true for the proportion between "level one" and the"interruption level". Therefore, there is no interest in transferringthe optical levels exactly as those happen to exist in the input signal.It is sufficient if the difference in levels is sufficiently large.There is also no interest in detecting the interruption level bymeasurement of the level since there is in reality no guarantee that"level zero" is higher than the "interruption level", even if it is inmost cases The only way to know for certain that a real interruption ispresent is to measure the time when the signal is not at "level one"since it is at such time that all data signals have a longest possiblelength of "level zero". The interruption detector 3 therefore serves toprimarily detect only "level one" or "not level one" conditions.

Thus the interruption detection delay provides that normal stops(depending on a number of "zeros" in the data signal) will always beshorter than the preset interruption detection delay. This delay isnecessary in order that the laser not be fully cut off at every optical"zero". The interruption detector 3 has an output signal which may stayin either of two logical states which are referred to herein as "freeze"and "non-freeze". When the interruption detector 3 detects aninterruption (after the delay) the output changes to the "freeze" state.As soon as the interruption is over it changes, after an arbitrary shortdelay, to the "non-freeze" state. One embodiment of the interruptiondetector 3 includes an adjustable interruption detection delay which isviewable from the outside of the interruption detector. The benefit ofthis arrangement is that the user of the device according to theinvention may interface the device to different types of optical inputsignals.

The low pass filter 4 filters the low frequency of the signal emitted byreceiver 2. The low pass filter 4 has a sufficiently high upperfrequency that overmodulated low frequency signals might be passed. Atthe same time the cut-off frequency must be sufficiently low such thatits output may be used as a decision threshold for a comparator 5. Thelow pass filter 4 also receives a logic input signal from theinterruption detector 3. If this logic input signal changes over to the"freeze" state the low pass filter immediately starts operating as ananalogue memory, i.e. the output of the low pass filter will keep theanalogue value it had at its output immediately before the "freeze"state was initiated. Note that the expression "analogue memory" bynecessity does not imply that the analogue memory consists of analoguecomponents. What is referred to is that an analogue signal value is keptin a memory which in principle may also contain digital components. Whenthe logic state is changed from "freeze" to "non-freeze", the low passfilter again immediately starts operating as an ordinary low passfilter. An embodiment of the low pass filter is depicted in FIG. 3. Ananalogue switch 16 is closed in the "non-freeze" state and opened in the"freeze" state. The real low pass filter consists of a resistance 14 anda capacitance 15 connected to signal ground 17. When the switch isclosed a normal one pole low pass filter is achieved and buffered by ahigh impedance amplifier 18 having typically unity gain. In the "freeze"state the analogue switch is opened and the analogue value presentbefore the "freeze" state is stored in the capacitor 15. Due to theamplifier having high impedance the analogue value may persist for along period of time. The analogue switch 16 is preferably an electronictype, e.g., a DMOS transistor.

An embodiment of a resistance 14 is demonstrated in FIG. 4. Theresistance 14 consists of a number of resistors which may be combined bymeans of analogue switches, which may be mechanical or electronic type.The resulting value of resistance between node 9 and node 19 may bevaried within wide limits. Accordingly, the cut-off frequency of the lowpass filter may be varied, and this is advantageous if the deviceaccording to the present invention has to be interfaced to differenttypes of optical input signals.

The comparator 5 comprises a decision circuitry. The task of suchcircuitry is to compare its two input signals, decide which input signalis the largest, and control its output signal such that it presents ahigh or a low output level depending upon which input signal is thelargest. During an interruption of the optical input signal, the firstupper input signal to the comparator 5 gets the value zero or a valuevery close to zero, while the second lower input signal, as previouslyshown, keeps the value present before the interruption. When the signalcomes back, the second lower input signal immediately has the correctlevel due to the low pass filter acting as an analogue memory for thesignal before the "freeze" state. When there is a normal input signalhaving about equal amounts of ones and zeros and no interruptions, thesecond lower input signal from the low pass filter will be approximatelyat the middle of the extreme values of the signal from the receiver 2,where one extreme corresponds to optical "one" and the other extremecorresponds to optical "zero". This is what is sought; in order for thedevice of FIG. 2 to operate well, over a long period of time (withinwhich interruptions are not counted) compared to the time constant ofthe low pass filter, the numbers of "ones" and "zeros" will beapproximately equal.

An AGC circuitry 6 is an amplifier having a very slow (order of Hz) gaincontrol, which slowly controls its gain such that the average value ofthe output signal is constant. The value of the output signal in turndetermines the optical output power from the laser, i.e. the outputsignal from the AGC circuitry 6 constitutes a set point for the controlloop of the laser transmitter. The AGC circuitry is provided since it isdesirable that the laser have the same output power level independent ofthe optical input power level. If different installations are compared,the optical input power level may vary by more than 10 dB dependent uponthe fibre attenuation and the number of optical terminals and the like.For a given installation the optical input power level is fairlyconstant if it is disregarded that the power decreases at interruptions.Due to the analogue memory function of the low pass filter 4, the gainof the AGC circuitry 6 will not be affected during an interruption.

A laser transmitter 7 has three input signals which include the signalfrom the comparator 5, the signal from the AGC circuitry and the logicalsignal from the interruption detector 6. The optical output signal fromthe laser transmitter is coupled to a fibre 8. The signal from thecomparator 5 modulates the laser by a high frequency data signal. Thesignal from the AGC circuitry 6 constitutes the set point for thecontrol loop of the laser transmitter. Like most conventional lasertransmitters for fibre optics, the laser transmitter 7 has a controlloop for the average power. What in reality is controlled in this andmost other laser transmitters is the light power incident on a monitordiode which is a photodiode positioned in proximity of the laser. Sincethe light being coupled to the fibre 8 is proportional to the lightincident of the monitor diode, the optical output signal to the fibre iscontrolled. Contradictory to most conventional laser transmitters,compared to the data signal, the set point constitutes a slowly varyingsignal. This signal has been passing the low pass filter 4 and the AGCcircuitry 6. It is important that the set value vary in time in order tobe able to transfer an overmodulated low frequency signal. So long asthe low pass filter 4 has a sufficiently large bandwidth to pass theovermodulated low frequency signal from the receiver 2, the average ofthe signal at node 13 will have an overmodulated low frequency signalwhich was initially present in the optical input signal according to theexample in FIG. 1. In order for the device according to the presentinvention to work in the intended way, it is important that the controlloop of the laser transmitter have a sufficiently large bandwidth to beable to transfer the overmodulated low frequency signal. As the logicsignal at node 10 changes from the "non-freeze" state to the "freeze"state two things happen simultaneously at the laser transmitter. First,the laser current is diverted, e.g., by a switch shorting out the laserdiode, such that the optical output power into the fibre goes to theinterruption level described previously. Secondly, the control loop ofthe laser transmitter is simultaneously "frozen" such that the analoguevalue controlling the average power of the laser is kept in an analoguememory. As noted above, the expression "analogue memory" does notnecessarily mean that the analogue memory consists of analoguecomponents. What is referred to is that an analogue signal value may bekept in a memory which in principle may also contain digital components.When the logic signal at node 10 returns to the "non-freeze" state, twothings happen simultaneously. First the diversion of the laser currentis discontinued. Secondly, the "freezing" of the control loop of thelaser is discontinued, i.e. it continues as usual having the analoguevalue from the analogue memory as a starting point. Due to the analoguememory function, the laser transmitter operates immediately after theinterruption. Without the analogue memory value it would take a longtime for the control loop of the laser to regulate into its normalvalue. An embodiment of the laser transmitter is depicted in FIG. 5.Assuming that a large optical signal from fibre 1 is giving a certainlogic level at node 12, the signal at node 12 will cause a currentgenerator 20 to supply a current which may be denoted I₁. At low opticalsignal in fibre 1 the level at node 12 shifts, which in turn decreasesthe current from the current generator 20 to a current I₀ which is lessthan I₁. The difference current I₁ -I₀ forms the peak-to-peak value ofthe high frequency modulation current passing a laser diode 21. It isimportant that the current difference I₁ -I₀ be sufficiently large todrive the laser diode from an optical level below or at the laserthreshold ("level zero") to the high optical level corresponding to anoptical "one" ("level one"). Note that the plus sign above the laserdiode 21 symbolizes that the laser is forward biased, i.e., that theanode is positive relative to the cathode. Also note that to simplifythe drawing the connection of the fibre 8 to the laser diode is notdrawn in FIG. 5. The laser diode has two mirrors emitting light. Thelight from one mirror is coupled to a monitor diode 22 and the lightfrom the other mirror is coupled to the fibre 8. The light impinging onthe monitor diode 22 generates a current which is proportional to thelevel of light. This current generates a signal proportional to thecurrent by means of a block 23 which may be, for example, a simpleresistor. This signal is connected to the negative input terminal of adifferential amplifier 24, which receives at its positive input terminal(node 13) the set point of the control loop. At its output thedifferential amplifier produces the difference signal between theinputs, i.e., the signal at the positive input minus the signal at thenegative input. The output signal from the differential amplifier 24 isconnected to an integrator 25, which, except for true integration, alsogives a certain gain which is denoted A. The bandwidth of the controlloop is directly proportional to the gain factor A according to classiccontrol theory. The output signal from the integrator 25 acts on acurrent generator 26, and when the current increases the optical powerinto the fibre 8 also increases. It should be noted that which has sofar been stated regarding this embodiment of the laser transmitter isknown in the art and has only been added as background for the rest ofthe description of the embodiment of the laser transmitter 7. As node 10changes to the "freeze" state, the integrator 25 keeps its analogueoutput signal which it had just before the "freeze" state occurred.Exactly simultaneously a switch 27 shorts out the laser diode 21 suchthat no, or almost no, current passes the laser diode. The switch 27 isan electronic switch, e.g., in form of a DMOS transistor. When node 10returns to the "non-freeze" state the integrator 25 stops acting as ananalogue memory and carries on simultaneously as the shorting of thelaser diode ceases. Due to this method, at the change from the "freeze"state to the "non-freeze" state, the laser diode may at once supply thecorrect optical output power without having to wait for the slowbuilding-up of the laser control loop. An embodiment of the integrator25 is depicted in FIG. 6. An amplifier 30 amplifies and inverts thesignal from node 28. an operational amplifier 34, a resistor 32 and acapacitor 33 forms a classic inverting integrator. As input 10 changesto the "freeze" state a switch 31 is opened which indicates that theanalogue value is stored at the output of the operational amplifier 34,provided that the operational amplifier 34 has high impedance inputs.The switch 31 is an electronically designed analogue switch. In oneembodiment of the amplifier 30, the user may, from the outside, vary thegain in order to vary the bandwidth of the control loop. This isadvantageous if the device according to the present invention has to beinterfaced to different types of optical input signals.

I claim:
 1. A device for conversion of an incoherent optical data signaltransmitted from a multimode fiber to a corresponding signal which istransmitted to a single mode fiber by a lasertransmitter,comprising:means connected to a multimode fiber fortransforming an optical input signal received from said multimode fiberinto an electric signal, said transforming means comprising a DC-coupledoptical receiver having a receiver input connected to said multimodefiber and a receiver output; a laser transmitter having a firsttransmitter input, a second transmitter input, a third transmitter inputand a transmitter output, said transmitter output being connected to asingle mode fiber and comprising a control loop and a memory deviceconnected thereto; an AGC circuit having an AGC input, and an AGC outputconnected to said first transmitter input; a low pass filter having afirst filter input connected to said receiver output, a second filterinput, and a filter output connected to said AGC input; comparator meansfor receiving said electric signal from said transforming means and fortransmitting a signal to said laser transmitter to modulate said lasertransmitter, said comparator means comprising a comparator havingdecision circuitry and a first comparator input connected to saidreceiver output, a second comparator input connected to said filteroutput and a comparator output connected to said second transmitterinput; and interruption detector means connected to said transformingmeans, said low pass filter and said laser transmitter (a) for detectingoptical interruption of said incoherent optical data signal and inresponse simultaneously (1) diverting laser current to interrupt opticaloutput power emitted into said single mode fiber and (2) storing ananalogue value of said control loop in said memory device, and (b) fordetecting when said optical interruption ceases and in responsesimultaneously (1) discontinuing diverting laser current to therebyresume said optical output power emitted into said single mode fiber and(2) operating said laser transmitter on the basis of analogue valuespreviously stored in said memory device, said interruption detectionmeans comprising a detection input connected to said receiver output anda detection output connected to said second filter input and said thirdtransmitter input.
 2. The device according to claim 1 wherein saidinterruption detector means is a preset interruption detection delaydevice.
 3. The device according to claim 1 wherein said interruptiondetector means is an adjustable interruption detection delay device. 4.The device according to claim 1 wherein said low pass filter comprises aconventional one pole RC filter having an analogue switch in series witha resistance and a high impedance amplifier buffering the filter.
 5. Thedevice according to claim 4 wherein said control loop of said lasertransmitter comprises a means for adjusting the bandwidth.
 6. The deviceaccording to claim 5 wherein said laser transmitter comprises anintegrator (25) comprising an amplifier (30), an operational amplifier(34) having a capacitance (33) between a negative input of saidoperational amplifier and the output of said operational amplifier, anda resistance (32) connected in series with an analogue switch (31)between an output of the amplifier and the negative input of saidoperational amplifier, the positive input of said operational amplifierbeing connected to a reference voltage.
 7. A method for conversion of anincoherent optical data signal transmitted from a multimode fiber to acorresponding signal which is transmitted to a single mode fiber by alaser transmitter, comprising the steps of:emitting an optical signal;transforming said optical signal into an electric signal and couplingsaid electric signal to a comparator to a low loss filter and to aninterruption detector; filtering said optical signal by means of saidlow loss filter and transmitting a resulting low loss filter signal tosaid comparator and an AGC circuit; modulating a laser transmitter bymeans of said comparator; outputting a set point signal for a controlloop of said laser transmitter by means of said AGC circuit; detectingoptical interruption of said incoherent optical data signal and inresponse simultaneously (1) diverting laser current to interrupt opticaloutput power emitted into said single mode fiber and (2) storing ananalogue value of said control loop in said memory device, and detectingwhen said optical interruption ceases and in response simultaneously (1)discontinuing diverting laser current to thereby resume said opticaloutput power emitted into said single mode fiber and (2) operating saidlaser transmitter on the basis of analogue values previously stored insaid memory device.